My two rats often play wrestle, but last night it apparently escalated. I heard Dawn squeaking louder than usual, but didn't think anything about it; but this morning half her tail was missing and she's sooo scared! What in the world would cause my other rat to do this? They had more than enough food. Dusk (the cannibal) has always been very timid and shy, where Dawn would explore and run around Dusk always stayed close to the cage. Having said that, Dusk has bitten me before...
It could be also that the tail got stuck somewhere between the bars if you have bars in your cage and then was torn off while they were wrestling. Have you found the missing tail somewhere?
Have your rats known each other for a long time, like a few months? Because where you introduce a new rat to the group, you have to do it carefully, especially if the new rat is not so young already (like a few weeks), watch them all the time and if you see aggression, blood, then give them a moment in separate cages to rethink and try to bring them together later.
If they are rats that have known each other for a longer period it is less likely for them to fight this kind of way, unless one of them is really bad tempered - then I think you should react when you hear screams from the cage, because fighting is OK but as long as there's not blood.
I found a site that states:
On close inspection, it can be seen that rats' tails are skin, soft with hair, and the "scale" is only a texture. Rat tails are easily injured and can tear from a bite.
My best guess as to what happened is they were either playing or got aggressive with one another and either accidentally or purposely bit off the tail of the other rat. Since you said that they often play wrestle, perhaps it was an accident, or the event escalated to the point where they began to actually fight.